2.7 Effluent Disposal and Beneficial Use

2.7.1 What are the options for effluent disposal?

Treated effluent from a wastewater treatment plant is normally
discharged into the nearest water body capable of accepting it without
detrimentally affecting it. This can be a drainage ditch, a river, stream or
torrent, a lake or the sea.

Fig. 2.5 Agricultural re-use of
treated effluent

In some cases, to ensure adequate and effective dilution of the
effluent in the receiving water, an underwater pipeline must be constructed,
equipped at its discharge end with a diffuser system. This is particularly so,
when treated effluent is discharged into the sea and acceptable bacterial levels
are required at the shoreline and inshore waters without resorting to
disinfection. The issue of mixing zones is important and the
environmental/economic balance of several small discharges versus one large
discharge needs careful assessment.

In addition to providing dilution, a long offshore outfall
provides time for natural bacterial die-off to occur before the considerably
diluted effluent reaches the shoreline. Techniques exist for predicting the
dilution, dispersion and die-off that will be achieved.

2.7.2 Under what circumstances is it worthwhile to reuse
treated effluent for beneficial use?

Recycling treated effluent for further use is unlikely to be
either necessary or worthwhile where natural water resources are sufficient to
satisfy all normal demands placed upon them by an area, e.g. satisfactorily
serving the needs of the population, commerce and industry, the public services,
landscaping and agriculture.

However, if water resources are either periodically or
continually unable to satisfy water demands in the area, the recycling of
treated effluent for beneficial use should be considered.

It is generally possible to reuse effluent for use as irrigation
water in agriculture, landscaping and forestry, although, unless it has been
either disinfected or stored for some time, care should be taken in its use on
crops eaten raw, or where spray irrigation techniques are used.

Treated sewage effluent can be used for secondary industrial
purposes such as cooling and quenching without treatment other than disinfection
and dosing with algaecides.

Some water deficient areas have insisted on dwellings and public
buildings being provided with dual plumbing systems for the water supply, one
for toilet flushing and the other for all other water uses. It is possible to
use treated effluent in the toilet flushing system as long as suitable
precautions have been taken against cross-connection with the potable supply,
e.g. colour coding of pipes and labelling of the effluent, and the
effluent has been disinfected and dosed with algaecide.

If other uses are under consideration, such as in industrial
processes or for drinking purposes, it will be necessary to subject the effluent
to considerable further treatment as for any other primary quality potable
supply. As effluent is generally of lower quality than natural water sources,
this can be very expensive and has been practised in very few
locations.